Closed-loop flow-injection analysis for the determination of a strong acid and base using an acetate buffer solution and methyl orange

The use of a closed-loop system in flow-injection analysis (FIA) allows for the recycling of reagents which are present tn excess. Strong acids and bases are determined by injection into a continually circulated buffer solution containing an acid-base indicator. Neutralization occurs in the sample zone and discoloration of an indicator is monitored by specirophoiometry. Passing though the flow cell, the resulting solution is then buffered and returns to the reservoir. A typical aqueous reservoir solution contained 0.1 M acetate buffer and 1.5 × 10 -4 M Methyl Orange. This solution was circulated at a flow rate of 2.5 ml/min. Sample (acid or base) aliquots of 2 μl were introduced into the stream and the differential absorbances were monitored at 530 nin. The calibration curve of samples by this method showed good linearity over a range of 0.1-5 M. Acids (HC1, HNO 3 , H 2 SO 1 , and oxalic acid) and bases (NaOH, KOH and NHs) gave the same slope of the calibration curve, respectively. Although a slight gradual increase in the baseline shift could not be avoided, it allowed 500 repetitive determinations of 1 M HC1 or NaOH with the same 100 ml of circulating 0.1 M acetate buffer solution.